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DP IB Chemistry: HL

Revision Notes

Home / IB / Chemistry: HL / DP / Revision Notes / 15. HL Energetics / Thermochemistry / 15.2 Entropy & Spontaneity / 15.2.1 Entropy


15.2.1 Entropy


Entropy

Entropy

  • You may have wondered why it is that endothermic reactions occur at all, after all, what can be the driving force behind endothermic reactions if the products end up in a less stable, higher energy state?
  • Although the majority of chemical reactions we experience everyday are exothermic,  ΔHꝋ alone is not enough to explain why endothermic reactions occur

5-1-2-endo-reaction-profile

The driving force behind chemical reactions cannot be explained by enthalpy changes alone as it does not sense for chemical to end up in a less stable higher energy state in endothermic reactions

  • The answer is entropy

Predicting Entropy Changes

Chaos in the universe

  • The entropy (S) of a given system is the number of possible arrangements of the particles and their energy in a given system
    • In other words, it is a measure of how disordered or chaotic a system is

  • When a system becomes more disordered, its entropy will increase
  • An increase in entropy means that the system becomes energetically more stable
  • For example, during the thermal decomposition of calcium carbonate (CaCO3) the entropy of the system increases:

CaCO3(s) → CaO(s) + CO2(g)

    • In this decomposition reaction, a gas molecule (CO2) is formed
    • The CO2 gas molecule is more disordered than the solid reactant (CaCO3), as it is constantly moving around
    • As a result, the system has become more disordered and there is an increase in entropy

  • Another typical example of a system that becomes more disordered is when a solid melts
    • For example, melting ice to form liquid water:

H2O(s) → H2O(l)

    • The water molecules in ice are in fixed positions and can only vibrate about those positions
    • In the liquid state, the particles are still quite close together but are arranged more randomly, in that they can move around each other
    • Water molecules in the liquid state are therefore more disordered
    • Thus, for a given substance, the entropy increases when its solid form melts into a liquid
  • In both examples, the system with the higher entropy will be energetically favourable (as the energy of the system is more spread out when it is in a disordered state)

Entropy change, downloadable AS & A Level Chemistry revision notes

Melting a solid will cause the particles to become more disordered resulting in a higher entropy state



  • 1. Stoichiometric Relationships
    • 1.1 Matter, Chemical Change & the Mole Concept
      • 1.1.1 Elements, Compounds & Mixtures
        • 1.1.2 Equations
          • 1.1.3 State Changes
            • 1.1.4 The Mole Concept
              • 1.1.5 Moles-Mass Problems
                • 1.1.6 Empirical Formulae
                • 1.2 Reacting Masses & Volumes
                  • 1.2.1 Reacting Masses
                    • 1.2.2 Reaction Yields
                      • 1.2.3 Avogadro's Law & Molar Gas Volume
                        • 1.2.4 The Ideal Gas Equation
                          • 1.2.5 Gas Law Relationships
                            • 1.2.6 Real Gases
                              • 1.2.7 Standard Solutions
                                • 1.2.8 Concentration Calculations
                                  • 1.2.9 Titrations
                                • 2. Atomic Structure
                                  • 2.1 Atomic & Electronic Structure
                                    • 2.1.1 The Nuclear Atom
                                      • 2.1.2 Deducing Subatomic Particles
                                        • 2.1.3 Relative Atomic Mass Calculations
                                          • 2.1.4 The Electromagnetic Spectrum
                                            • 2.1.5 Emission Spectra
                                              • 2.1.6 Energy Levels & Sublevels
                                                • 2.1.7 Sublevels & Orbitals
                                                  • 2.1.8 Writing Electron Configurations
                                                • 3. Periodicity
                                                  • 3.1 The Periodic Table & Periodic Trends
                                                    • 3.1.1 The Periodic Table
                                                      • 3.1.2 Periodic Trends: Physical - Atomic & Ionic Radius
                                                        • 3.1.3 Periodic Trends: Physical - Ionisation Energy
                                                          • 3.1.4 Periodic Trends: Physical - Electron Affinity
                                                            • 3.1.5 Periodic Trends: Physical - Electronegativity
                                                              • 3.1.6 Periodic Trends: Chemical
                                                              • 3.2 Oxides, Group 1 & Group 17
                                                                • 3.2.1 Periodic Trends: Oxides Across a Period
                                                                  • 3.2.2 Periodic Trends: Group 1 - The Alkali Metals
                                                                    • 3.2.3 Periodic Trends: Group 17 - The Halogens
                                                                  • 4. Chemical Bonding & Structure
                                                                    • 4.1 Ionic & Covalent Bonding
                                                                      • 4.1.1 Forming Ions
                                                                        • 4.1.2 Ionic Compounds
                                                                          • 4.1.3 Formulae & Names of Ionic Compounds
                                                                            • 4.1.4 Covalent Bonds
                                                                              • 4.1.5 Bond Polarity
                                                                                • 4.1.6 Lewis Structures
                                                                                • 4.2 Resonance, Shapes & Giant Structures
                                                                                  • 4.2.1 Resonance Structures
                                                                                    • 4.2.2 Shapes of Molecules
                                                                                      • 4.2.3 Predicting Molecular Shapes
                                                                                        • 4.2.4 Molecular Polarity
                                                                                          • 4.2.5 Giant Covalent Structures
                                                                                          • 4.3 Intermolecular Forces & Metallic Bonding
                                                                                            • 4.3.1 Types of Intermolecular Forces
                                                                                              • 4.3.2 Deducing Intermolecular Forces
                                                                                                • 4.3.3 Properties of Covalent Compounds
                                                                                                  • 4.3.4 Metallic Bonding
                                                                                                    • 4.3.5 Trends in Melting Points of Metals
                                                                                                      • 4.3.6 Alloys & their Properties
                                                                                                    • 5. Energetics/ Thermochemistry
                                                                                                      • 5.1 Energetics
                                                                                                        • 5.1.1 Energy Changes in Reactions
                                                                                                          • 5.1.2 Standard Enthalpy Change
                                                                                                            • 5.1.3 Calorimetry
                                                                                                              • 5.1.4 Calorimetry Experiments
                                                                                                              • 5.2 Hess's Law
                                                                                                                • 5.2.1 Hess's Law
                                                                                                                  • 5.2.2 Using Hess's Law - Cycles
                                                                                                                    • 5.2.3 Using Hess's Law - Equations
                                                                                                                      • 5.2.4 Using ∆Hf° to Find Enthalpy Changes
                                                                                                                      • 5.3 Bond Enthalpy
                                                                                                                        • 5.3.1 Bond Enthalpy
                                                                                                                          • 5.3.2 Bond Enthalpy Calculations
                                                                                                                            • 5.3.3 Energy Profiles
                                                                                                                              • 5.3.4 Case Study: Ozone
                                                                                                                            • 6. Chemical Kinetics
                                                                                                                              • 6.1 Chemical Kinetics
                                                                                                                                • 6.1.1 Collision Theory
                                                                                                                                  • 6.1.2 Rate of Reaction
                                                                                                                                    • 6.1.3 Measuring Rates
                                                                                                                                      • 6.1.4 Activation Energy
                                                                                                                                        • 6.1.5 Rate Experiments
                                                                                                                                          • 6.1.6 Explaining Rates
                                                                                                                                            • 6.1.7 Maxwell-Boltzmann Curves
                                                                                                                                              • 6.1.8 Energy Profiles & Catalysis
                                                                                                                                            • 7. Equilibrium
                                                                                                                                              • 7.1 Equilibrium
                                                                                                                                                • 7.1.1 The State of Equilibrium
                                                                                                                                                  • 7.1.2 The Equilibrium Law
                                                                                                                                                    • 7.1.3 Equilibrium Constant Relationships
                                                                                                                                                      • 7.1.4 The Reaction Quotient
                                                                                                                                                        • 7.1.5 Le Chatelier's Principle
                                                                                                                                                          • 7.1.6 Catalysts & Equilibrium
                                                                                                                                                        • 8. Acids & Bases
                                                                                                                                                          • 8.1 Theories & Reactions of Acids & Bases
                                                                                                                                                            • 8.1.1 Brønsted–Lowry Acids & Bases
                                                                                                                                                              • 8.1.2 Conjugate Acid-Base Pairs
                                                                                                                                                                • 8.1.3 Characteristic Reactions of Acids
                                                                                                                                                                  • 8.1.4 Neutralization
                                                                                                                                                                  • 8.2 More About Acids
                                                                                                                                                                    • 8.2.1 Acid-base Titrations
                                                                                                                                                                      • 8.2.2 pH & [H⁺]
                                                                                                                                                                        • 8.2.3 Interpreting pH
                                                                                                                                                                          • 8.2.4 The Ionic Product of Water
                                                                                                                                                                            • 8.2.5 Acid-Base Calculations
                                                                                                                                                                              • 8.2.6 pH Meters & Universal Indicator
                                                                                                                                                                                • 8.2.7 Strong & Weak Acids & Bases
                                                                                                                                                                                  • 8.2.8 Comparing Strong & Weak Acids
                                                                                                                                                                                  • 8.3 Acid Deposition
                                                                                                                                                                                    • 8.3.1 Acid Deposition
                                                                                                                                                                                      • 8.3.2 Effects of Acid Deposition
                                                                                                                                                                                        • 8.3.3 Reducing Sulfur Oxide Emissions
                                                                                                                                                                                      • 9. Redox Processes
                                                                                                                                                                                        • 9.1 Redox Processes
                                                                                                                                                                                          • 9.1.1 Oxidation & reduction
                                                                                                                                                                                            • 9.1.2 Deducing Oxidation Numbers
                                                                                                                                                                                              • 9.1.3 Oxidizing & Reducing agents
                                                                                                                                                                                                • 9.1.4 Naming Transition Metal Compounds
                                                                                                                                                                                                  • 9.1.5 Half Equations
                                                                                                                                                                                                    • 9.1.6 The Activity Series
                                                                                                                                                                                                      • 9.1.7 Redox Titrations
                                                                                                                                                                                                        • 9.1.8 The Winkler Method
                                                                                                                                                                                                          • 9.1.9 Voltaic Cells
                                                                                                                                                                                                            • 9.1.10 Electrolytic Cells
                                                                                                                                                                                                          • 10. Organic Chemistry
                                                                                                                                                                                                            • 10.1 Fundamentals of Organic Chemistry
                                                                                                                                                                                                              • 10.1.1 Homologous Series
                                                                                                                                                                                                                • 10.1.2 Understanding Organic Molecules
                                                                                                                                                                                                                  • 10.1.3 Nomenclature
                                                                                                                                                                                                                    • 10.1.4 Organic Families - Hydrocarbons
                                                                                                                                                                                                                      • 10.1.5 Organic Families - Halogenoalkanes
                                                                                                                                                                                                                        • 10.1.6 Organic Families - Alcohols & Ethers
                                                                                                                                                                                                                          • 10.1.7 Organic Families - Carbonyls
                                                                                                                                                                                                                            • 10.1.8 Organic Families - Carboxylic Acids & Esters
                                                                                                                                                                                                                              • 10.1.9 Organic Families - Organic Nitrogen Compounds
                                                                                                                                                                                                                                • 10.1.10 3-D modelling
                                                                                                                                                                                                                                  • 10.1.11 Primary, Secondary & Tertiary atoms
                                                                                                                                                                                                                                    • 10.1.12 Benzene
                                                                                                                                                                                                                                    • 10.2 Functional Group Chemistry
                                                                                                                                                                                                                                      • 10.2.1 Alkanes - Combustion
                                                                                                                                                                                                                                        • 10.2.2 Alkanes - Halogenation
                                                                                                                                                                                                                                          • 10.2.3 Alkenes - Reactivity
                                                                                                                                                                                                                                            • 10.2.4 Alkenes - Hydrogenation
                                                                                                                                                                                                                                              • 10.2.5 Alkenes - Halogenation
                                                                                                                                                                                                                                                • 10.2.6 Alkenes - Hydrohalogenation
                                                                                                                                                                                                                                                  • 10.2.7 Alkenes - Hydration
                                                                                                                                                                                                                                                    • 10.2.8 Addition Polymers
                                                                                                                                                                                                                                                      • 10.2.9 Alcohols - Combustion
                                                                                                                                                                                                                                                        • 10.2.10 Alcohols - Oxidation
                                                                                                                                                                                                                                                          • 10.2.11 Alcohols - Esterification
                                                                                                                                                                                                                                                            • 10.2.12 Halogenoalkanes
                                                                                                                                                                                                                                                              • 10.2.13 Reactions of Benzene
                                                                                                                                                                                                                                                            • 11. Measurements & Data Processes
                                                                                                                                                                                                                                                              • 11.1 Spectroscopic Identification
                                                                                                                                                                                                                                                                • 11.1.1 Index of Hydrogen Deficiency
                                                                                                                                                                                                                                                                  • 11.1.2 Mass Spectrometry
                                                                                                                                                                                                                                                                    • 11.1.3 Nuclear Magnetic Resonance Spectroscopy
                                                                                                                                                                                                                                                                      • 11.1.4 Infrared Spectroscopy
                                                                                                                                                                                                                                                                      • 11.2 Synoptic Data Handling & Graphical Skills
                                                                                                                                                                                                                                                                        • 11.2.1 Recording Data
                                                                                                                                                                                                                                                                          • 11.2.2 Processing Errors
                                                                                                                                                                                                                                                                            • 11.2.3 Reducing the Impact of Errors
                                                                                                                                                                                                                                                                              • 11.2.4 Percentage Error
                                                                                                                                                                                                                                                                                • 11.2.5 Drawing Graphs
                                                                                                                                                                                                                                                                                  • 11.2.6 Analysing Graphs
                                                                                                                                                                                                                                                                                • 12. HL Atomic Structure
                                                                                                                                                                                                                                                                                  • 12.1 Electrons in Atoms
                                                                                                                                                                                                                                                                                    • 12.1.1 Emission Spectra & First Ionisation Energy
                                                                                                                                                                                                                                                                                      • 12.1.2 Ionisation Energy Trends
                                                                                                                                                                                                                                                                                    • 13. HL The Periodic Table- Transition Metals
                                                                                                                                                                                                                                                                                      • 13.1 Transition Metals
                                                                                                                                                                                                                                                                                        • 13.1.1 Transition Elements
                                                                                                                                                                                                                                                                                          • 13.1.2 Variable Oxidation States
                                                                                                                                                                                                                                                                                            • 13.1.3 Complex Ions
                                                                                                                                                                                                                                                                                              • 13.1.4 Catalytic & Magnetic Properties
                                                                                                                                                                                                                                                                                                • 13.1.5 Colour in Transition Metals
                                                                                                                                                                                                                                                                                                  • 13.1.6 Factors Affecting Colour
                                                                                                                                                                                                                                                                                                • 14. HL Chemical Bonding & Structure
                                                                                                                                                                                                                                                                                                  • 14.1 More Structures & Shapes
                                                                                                                                                                                                                                                                                                    • 14.1.1 Covalent Bonds
                                                                                                                                                                                                                                                                                                      • 14.1.2 More Lewis Structures
                                                                                                                                                                                                                                                                                                        • 14.1.3 Further VSEPR Theory
                                                                                                                                                                                                                                                                                                          • 14.1.4 Formal Charge
                                                                                                                                                                                                                                                                                                          • 14.2 Further Aspects of Bonding
                                                                                                                                                                                                                                                                                                            • 14.2.1 Delocalisation & Resonance
                                                                                                                                                                                                                                                                                                              • 14.2.2 Ozone Revisited
                                                                                                                                                                                                                                                                                                                • 14.2.3 Hybrid Orbitals
                                                                                                                                                                                                                                                                                                              • 15. HL Energetics / Thermochemistry
                                                                                                                                                                                                                                                                                                                • 15.1 Energy Cycles
                                                                                                                                                                                                                                                                                                                  • 15.1.1 Key Enthalpy Terms
                                                                                                                                                                                                                                                                                                                    • 15.1.2 Born-Haber Cycles
                                                                                                                                                                                                                                                                                                                      • 15.1.3 Born-Haber Cycle Calculations
                                                                                                                                                                                                                                                                                                                        • 15.1.4 Dissolution Energy Cycles
                                                                                                                                                                                                                                                                                                                          • 15.1.5 Energy Changes in Aqueous Solutions
                                                                                                                                                                                                                                                                                                                          • 15.2 Entropy & Spontaneity
                                                                                                                                                                                                                                                                                                                            • 15.2.1 Entropy
                                                                                                                                                                                                                                                                                                                              • 15.2.2 Calculating Entropy Change
                                                                                                                                                                                                                                                                                                                                • 15.2.3 Gibbs Free Energy
                                                                                                                                                                                                                                                                                                                                  • 15.2.4 Spontaneous Reactions
                                                                                                                                                                                                                                                                                                                                    • 15.2.5 Free Energy & Equilibrium
                                                                                                                                                                                                                                                                                                                                  • 16. HL Chemical Kinetics
                                                                                                                                                                                                                                                                                                                                    • 16.1 Rate Expression & Reaction Mechanism
                                                                                                                                                                                                                                                                                                                                      • 16.1.1 Rate Equations
                                                                                                                                                                                                                                                                                                                                        • 16.1.2 Rate Constants
                                                                                                                                                                                                                                                                                                                                          • 16.1.3 Graphical Representations
                                                                                                                                                                                                                                                                                                                                            • 16.1.4 Rate Determining Step
                                                                                                                                                                                                                                                                                                                                              • 16.1.5 Mechanism Problems
                                                                                                                                                                                                                                                                                                                                              • 16.2 Activation Energy
                                                                                                                                                                                                                                                                                                                                                • 16.2.1 The Arrhenius Equation
                                                                                                                                                                                                                                                                                                                                                  • 16.2.2 Graphing the Arrhenius Equation
                                                                                                                                                                                                                                                                                                                                                    • 16.2.3 Activation Energy from Rate Constants
                                                                                                                                                                                                                                                                                                                                                  • 17. HL Equilibrium
                                                                                                                                                                                                                                                                                                                                                    • 17.1 The Equilibrium Law
                                                                                                                                                                                                                                                                                                                                                      • 17.1.1 Applying the Equilibrium Law
                                                                                                                                                                                                                                                                                                                                                        • 17.1.2 Gibbs Free Energy & the Equilibrium Constant
                                                                                                                                                                                                                                                                                                                                                      • 18. HL Acids & Bases
                                                                                                                                                                                                                                                                                                                                                        • 18.1 Further Aspects of Acids & Bases
                                                                                                                                                                                                                                                                                                                                                          • 18.1.1 Lewis Theory
                                                                                                                                                                                                                                                                                                                                                            • 18.1.2 pH Curves
                                                                                                                                                                                                                                                                                                                                                              • 18.1.3 Indicators
                                                                                                                                                                                                                                                                                                                                                                • 18.1.4 Buffers
                                                                                                                                                                                                                                                                                                                                                                  • 18.1.5 Salt Hydrolysis
                                                                                                                                                                                                                                                                                                                                                                  • 18.2 Calculations Involving Acids & Bases
                                                                                                                                                                                                                                                                                                                                                                    • 18.2.1 Acid & Base Dissociation Constants
                                                                                                                                                                                                                                                                                                                                                                      • 18.2.2 Acid & Base Problem Solving
                                                                                                                                                                                                                                                                                                                                                                    • 19. HL Redox Processes
                                                                                                                                                                                                                                                                                                                                                                      • 19.1 Electrochemical Cells
                                                                                                                                                                                                                                                                                                                                                                        • 19.1.1 Electrochemical Cells
                                                                                                                                                                                                                                                                                                                                                                          • 19.1.2 The Standard Hydrogen Electrode
                                                                                                                                                                                                                                                                                                                                                                            • 19.1.3 Electrolysis of Aqueous Solutions
                                                                                                                                                                                                                                                                                                                                                                              • 19.1.4 Quantitative Electrolysis
                                                                                                                                                                                                                                                                                                                                                                                • 19.1.5 Free Energy & Eº
                                                                                                                                                                                                                                                                                                                                                                                  • 19.1.6 Electroplating
                                                                                                                                                                                                                                                                                                                                                                                • 20. HL Organic Chemistry
                                                                                                                                                                                                                                                                                                                                                                                  • 20.1 Types of Organic Reactions
                                                                                                                                                                                                                                                                                                                                                                                    • 20.1.1 Nucleophilic Substitution Reactions
                                                                                                                                                                                                                                                                                                                                                                                      • 20.1.2 Electrophilic Addition Reactions
                                                                                                                                                                                                                                                                                                                                                                                        • 20.1.3 Electrophilic Substitution Reactions
                                                                                                                                                                                                                                                                                                                                                                                          • 20.1.4 Reduction Reactions
                                                                                                                                                                                                                                                                                                                                                                                          • 20.2 Synthetic Routes
                                                                                                                                                                                                                                                                                                                                                                                            • 20.2.1 Synthesis
                                                                                                                                                                                                                                                                                                                                                                                            • 20.3 Stereoisomerism
                                                                                                                                                                                                                                                                                                                                                                                              • 20.3.1 Stereoisomers
                                                                                                                                                                                                                                                                                                                                                                                                • 20.3.2 Cis-Trans & E/Z Isomers
                                                                                                                                                                                                                                                                                                                                                                                                  • 20.3.3 Optical Isomers
                                                                                                                                                                                                                                                                                                                                                                                                • 21. Measurement & Analysis
                                                                                                                                                                                                                                                                                                                                                                                                  • 21.1 Spectroscopic Identification of Organic compounds
                                                                                                                                                                                                                                                                                                                                                                                                    • 21.1.1 High Resolution NMR
                                                                                                                                                                                                                                                                                                                                                                                                      • 21.1.2 X-Ray Crystallography
                                                                                                                                                                                                                                                                                                                                                                                                        • 21.1.3 Structure Identification Problems
                                                                                                                                                                                                                                                                                                                                                                                                      Stewart Hird

                                                                                                                                                                                                                                                                                                                                                                                                      Author: Stewart

                                                                                                                                                                                                                                                                                                                                                                                                      Stewart has been an enthusiastic GCSE, A Level and IB Chemistry teacher for more than 30 years in the UK as well as overseas, and has been an examiner for IB and A Level, also. As a long-standing Head of Department, Stewart brings a wealth of experience to creating Topic Questions and revision materials for Save My Exams.


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